In a process of manufacturing a substrate such as a silicon wafer, a substrate cleaning process of an immersion type, a single wafer type or the like has been conventionally conducted in order to remove, from the substrate, organic substances, metallic impurities, particles, natural oxide films and the like that cause a defect in a semiconductor device.
In the substrate cleaning process, various types of cleaning methods have been used depending on its purpose. Particularly when the immersion-type cleaning method is used to remove foreign substances such as particles, use is made of a method for immersing the substrate in a cleaning liquid contained in a cleaning tank and irradiating the cleaning liquid in which the substrate is immersed with ultrasonic waves having a frequency of around 1 MHz, which is called megasonic. It is generally believed that when the ultrasonic waves having a frequency of around 1 MHz are used, damage to the substrate can be reduced and the cleaning effect on submicron-size fine particles on the substrate surface can be increased.
It has been known that a concentration of a dissolved gas in the cleaning liquid affects the efficiency of removal of the foreign substances such as particles. It has been found that when ultrapure water is used as the cleaning liquid and the ultrapure water is irradiated with megasonic to remove the particles from the substrate, for example, a rate of removal of the particles from the substrate is affected by a dissolved nitrogen concentration in the cleaning liquid. More specifically, when the dissolved gas concentration in the cleaning liquid is within a prescribed range, the rate of removal of the particles from the substrate is relatively high (Japanese Patent Laying-Open Nos. JP10-109072A and JP2007-250726A). Therefore, by monitoring the dissolved gas concentration such as the dissolved nitrogen concentration in the cleaning liquid and controlling the dissolved gas concentration in the cleaning liquid to fall within a certain range in the cleaning process, it becomes theoretically possible to remove the particles effectively.
On the other hand, there is a report that the behavior of removal of the particles from the substrate is somehow related to the behavior of weak light emission (sonoluminescence) that occurs when the cleaning liquid is irradiated with the ultrasonic waves (“Behaviour of a Well Designed Megasonic Cleaning System”, Solid State Phenomena Vols. 103-104 (2005) pp. 155-158; “Megasonics: A cavitation driven process”, Solid State Phenomena Vols. 103-104 (2005) pp. 159-162).